It is known to use cryosurgical instruments in surgical operations in which living tissue is destroyed (or ablated) by cooling the tissue to well below the freezing point of water (e.g. -20.degree. to -30.degree. Centigrade).
WO 93/04647 discloses a cryosurgical instrument in which a metal tip of a cryosurgical probe is cooled by supplying liquid nitrogen to the interior of the tip.
In such an instrument, when the liquid nitrogen reaches the interior of the metal tip, it starts to boil. There are two possible modes of boiling which can take place. If so-called `film` boiling occurs, a film of nitrogen gas forms between the inner surface of the metal tip and the boiling liquid nitrogen. This film of gas reduces the heat conduction between the boiling liquid and the body of the tip and thus reduces the cooling efficiency of the instrument. The other possible mode of boiling is referred to as `nucleate` boiling. In this mode the film of gas does not form, so the heat conduction between the boiling liquid and the body of the tip is much better, and the cooling efficiency of the tip is thus improved.
Although nucleate boiling provides a more efficient cooling operation, it is often difficult to control the mode of boiling reliably so that nucleate boiling takes place. In particular, film boiling often occurs for some time at the start of a period of operation of a cryosurgical instrument. Because the cooling efficiency is lower when film boiling takes place, surgical procedures in which tissue has to be cooled can take longer. This can be both hazardous (by increasing the time during which a patient must be anaesthetized), expensive and inconvenient.